Aggressive behavior in the house mouse, Mus musculus, is generally considered to be a sexually dimorphic character with the males far more aggressive than the females. Recent demonstrations of high levels of aggression in female mice appear to contradict this generalization. The proposed research will focus on differences in male and female responsiveness to environmental factors that influence the development and manifestation of aggressive behavior. The results of these experiments may reconcile the differences in published results and, more importantly, demonstrate that the capacity for aggressive behavior in mice is determined not by sex alone, but by the interaction of sex and various environmental factors. The first study will examine the influence of rearing conditions and stimulus animal type on the development of agrression in male and female mice. The remaining studies will investigate the role of gonadal hormones in one of the sex differences demonstrated in the first experiment. The second experiment will determine the role of gonadal integrity on the aggressive behavior of grouped males and females. If gonadectomy reduces the level of aggressive behavior in this study, the third experiment will examine the effectiveness of exogenous steroids in the activation of aggression in the two sexes. The final experiment will investigate the influence of neonatal androgen treatment on the development of the sex differences in the aggressive response to rearing conditions.